Over the centuries wind operated pipes have been developed to produce a wide variety of musical sounds, the pipe organ being a common application thereof. Pipe organs usually include a separate set or rank of pipes for each stop of the organ; thus, there might be one rank of 61 Diapason pipes (including one pipe for each note), and another rank of 61 Flute pipes, and another of 61 Trompette pipes, etc. Any particular rank can further be characterized as being made up of "flue" pipes or "reed" pipes. While there are almost countless varieties within each family, flue pipes are all essentially whistles, with the differences in tone color between one flue rank and another being determined by the relative dimensions of corresponding pipes, the material of which they are constructed, whether the pipes are open or stopped, and other factors.
Reed pipes generate tones in an entirely different manner, the primary source of tone being a vibrating brass reed which is coupled with a resonator for determining the characteristics of the tone. Over the long period of development of pipe organs, organ reed pipes have been successfully designed to be highly imitative of certain orchestral voices. However, because the natural speech characteristics of a reed, due to its mechanical construction and the nature of its operation, are not the same as the speech characteristics of the orchestral instruments they are intended to simulate, it has not been possible to make reeds sound exactly like orchestral voices. Organ reeds nonetheless have a certain quality and charm that cannot be duplicated by orchestral instruments, and it is the object of the present invention to provide an electronic organ that simulates the unique characteristics of that class of reed organ pipes that includes Kinura, Musette, Krummet and Vox Humana.
A reed organ pipe consists essentially of a reed tube having an opening, called a shallot, cut into one side thereof, against which a brass reed tongue is held by a tuning wire having spring tension, the reed and shallot being enclosed in a boot having an opening at the bottom, which communicates with a windchest to allow air into the pipe when its corresponding valve is operated, and a resonator coupled to the boot. Depending upon the desired organ voice, the resonator takes a variety of forms, such as a tapered, open-ended resonator for a chorus reed stop such as an organ Trompette, a partially closed flared pipe for obtaining an Oboe tone, a slightly tapered pipe with a slot in its wall near its remote end for a more imitative Orchestral Oboe, or capped resonators of various shapes and sizes to achieve other tonal effects. The resonators are usually scaled so as to be approximately one-half the wavelength of the sound of the note to be produced, but many interesting tone qualities are produced by pipes having short length resonators, as for example, the Kinura and Vox Humana.
In operation, the brass reed or tongue vibrates against the shallot, the reed being supported with respect to the shallot so that it essentially "rolls" past the shallot upon opening and closing so as to gradually open and close the opening; the size and shape of the reed and its curvature relative to the size and shape of the shallot determine the shape of the pulse of air that excites the resonator, which, in turn, determines the tonal quality. The relationship between the reed and the resonator also affect the tonal quality: if the resonator is tuned sharp or flat relative to the tuning reed, the resulting sound is either "choked" or "free", respectively. By appropriately adjusting the shape, configuration and size of the resonator used with a particular reed one can adjust the tone quality of a given pipe over a wide range, making it possible to make organ reed pipes highly imitative of certain orchestral voices. It is again emphasized, however, that the object of the present invention is not to simulate orchestral voices but, rather, to simulate by electronic circuit means the peculiar characteristics of certain reed pipes, thereby to imitate natural pipe organ voices.
The co-pending patent application Ser. No. 641,716 of Richard H. Peterson filed Dec. 17, 1975 which is now U.S. Pat. No. 4,023,455 and is commonly assigned, describes a system in which the speech characteristics of reed pipes are simulated by applying a train of electrical pulses rich in natural harmonics to a sharp cutoff low-pass filter having a sharp knee and a very rapid rate of roll-off, of the order of 24dB per octave or greater, above the cutoff frequency, the object being that as the frequency of the harmonics of the pulse signal increases, the amplitude of the output signal from the filter remains essentially constant up to the cutoff frequency and thereafter essentially immediately drops for frequencies above the cutoff frequency. Using a filter having these characteristics, the resultant output signal produces a sound which is surprisingly reed-like. To simulate the change in pitch that occurs in a reed pipe when the reed is partially open and as it is closing, particularly when it is just opening, the cutoff frequency of the filter is made automatically adjustable, from its lowest cutoff frequency when no keys of the organ are played to a higher cutoff frequency when a key is played, such higher cutoff frequency permitting several more harmonics to pass through the filter, the number being appropriate to produce the characteristic of a particular reed voice.
While the described Peterson system produces tonal effects highly imitative of the speech characteristics of many reed organ pipes, it is unable, because of their significantly different tonal characteristics, to provide the desired imitation of reed organ voices such as Kinura, Musette, Krummet and Vox Humana, the Vox Humana being the one most typically used. These voices, unlike those produced by most other reed organ pipes, have in addition to a cutoff characteristic a resonant characteristic which together produce the vowel-like sounds typical of this class of reed organ pipes. The reed vibrates at a fundamental frequency and the associated resonator, a high Q acoustic resonant circuit, is designed to strongly emphasize the fundamental and one of the higher order harmonics, and to greatly attenuate the harmonics falling in between. A similar vowel sound can be produced by processing a single pulse through a high Q resonant circuit, but this approach is prohibitively expensive because a separate resonant circuit, each with a different characteristic, is required for each note or small group of notes, to obtain the proper formant for each note of the musical scale.
The primary object of the present invention is to provide, at moderate cost, electronic circuit means for producing tonal effects highly simulative of Vox Humana and similar reed organ pipes.